In order to protect the environment, vapors which become separated from fuels in the fuel tank of a motor vehicle must not enter the environment. In order to prevent this from happening, motor vehicles are equipped with an activated carbon filter (ACF) which is intended to adsorb these vapors. However, since these activated carbon filters cannot adsorb and store an infinite quantity of fuel vapors, it is necessary to empty or regenerate the filters. For this purpose, modern motor vehicles have fuel tank venting systems, in which fuel that has evaporated in the fuel tank is stored in an activated carbon filter which is connected to the intake manifold of the internal combustion engine via a fuel tank vent valve which may be shut off. When the fuel tank vent valve is open, air is sucked in via a connection of the activated carbon filter to the surroundings, which entrains the temporarily stored fuel and feeds it to the combustion process. Via the fuel tank vent valve, the gas quantity taken in is controlled in such a way that on the one hand the activated carbon filter is sufficiently rinsed with air and on the other hand no intolerably large disruptions occur to the fuel/air ratio of the mixture fed to the internal combustion engine.
In motor vehicles which are to be sold in the United States of America, according to the California Air Resources Board (CARB) regulations a diagnosis must be carried out on all components of a motor vehicle which are responsible for emissions. This also includes a diagnosis of the fuel tank vent valve and of a pressure sensor in the fuel tank which is required in order to diagnose tank leakages. Furthermore, the individual components must be reliably identified in the event of a defect. For this purpose, it should be noted in particular that defective components may have an effect on correctly functioning components. This could lead to the correctly functioning components being incorrectly identified as defective. For example, a defective fuel tank vent valve should not lead to the tank pressure sensor being noted as defective, and vice versa. The individual components are therefore tested with the aid of diagnostic methods and are checked for plausibility in order to be able to find an error and assign it to a component. This test of the individual components usually takes place once per driving cycle. If a component defect appears only after it has been checked, the error need not necessarily be recognized immediately but rather must be recognized in the next driving cycle.
Presently known diagnostic methods include monitoring of the fuel tank vent valve by activating the fuel tank vent valve (so-called impressing) and evaluating the resulting signal by way of an additional downstream pressure sensor or by way of the pressure sensor in the intake manifold. Another method for monitoring the fuel tank vent valve provides for observing the gas mixture which reaches a lambda sensor in the exhaust tract of the motor vehicle.
German Published Patent Application No. 43 42 431 describes a method for ascertaining a piece of information about the state of a fuel tank venting system in an internal combustion engine, in which a degree of the effect of the evaporation of fuel within the fuel tank venting system on the occurring pressure change is ascertained, and this degree is taken into account when ascertaining the information about the state of the fuel tank venting system.